1. Field of the Invention
The present invention relates to an optical apparatus provided with an enlarged imaging optical apparatus such as a microscope and the like, and more particularly to an optical apparatus for automatically correcting various aberrations caused by the difference of the thickness of a medium for holding a sample, such as cover glass, a glass petri dish, a plastic container and the like, interposed between an objective and the specimen.
2. Description of the Related Art
Generally, a large magnification objective, such as a microscope and the like, is designed in such a way as to obtain a clear image when observing a fixed specimen using a piece of cover glass whose thickness and refractive index is regulated. Therefore, when observing using a piece of cover glass whose thickness and refractive index largely deviate from the standard ones, various aberrations occur to be unable to obtain a clear image. The larger the numerical aperture of an objective is, the more remarkable this tendency is.
Therefore, in some objectives, a part of a lens system constituting the objective can be moved along the optic axis. Such an objective is called an objective with a correction collar, which includes the following lenses. (1) A dry objective with a large numerical aperture in which various aberrations remarkably occur due to the uneven thickness of cover glass. (2) An objective supposed to use a piece of cover glass or a plastic container whose thickness widely differs, for example, in a fairly wide range of 0.17 mm to 2 mm. When using such objectives with a correction collar, a clear image can be obtained against various thickness of cover glass, compared with an objective without a correction collar.
However, actually it is not easy to correct aberration using such a microscope objective and only a skilled observer can find a position where an image can be the clearest. This is because a focus position also changes simultaneously as various aberrations change when moving a part of lenses for aberration correction. When the focus position changes, the image blurs. Since in a blurred image, the correction state of aberration cannot be caught, the observer of the microscope has to always repeat aberration correction and focusing. Therefore, the change in the amount of aberration cannot be continuously caught.
Japanese Patent Application No. 2002-169101 discloses a technology for automatically correcting the change of the focus position caused when moving the correction collar in order to solve the above-described problem. However, since the aberration cannot be automatically corrected, the convenience of the observer is not fully satisfied.
Japanese Patent Application No. 2001-083428 discloses an example of a television camera obtaining the image of an observed object mounted on a stage, the television camera outputting the image as electric signals, a first control unit driving a correction lens driver unit, based on the electric signals from the television camera to move a spherical aberration correction lens in a microscope device disposing the spherical aberration correction lens between the objective and an image forming unit and capable of correcting the spherical aberration by moving the spherical aberration correction lens along the optic axis. Since in this method, the correction lens driver unit is driven on the basis of the obtained image of an observed object, an exposure time for image acquisition become long especially when a weak fluorescent image is targeted. When repeated image picking is necessary, a far longer time is necessary. It is also necessary to apply excitation light to a sample in order to form an image, which gives damage due to light to the sample.